Which of these numbers is prime? ${27,\ 41,\ 85,\ 88,\ 99}$
A prime number has exactly two factors: 1 and itself. The factors of 27 are 1, 3, 9, and 27. The factors of 41 are 1 and 41. The factors of 85 are 1, 5, 17, and 85. The factors of 88 are 1, 2, 4, 8, 11, 22, 44, and 88. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 41 is a prime number.